Light sources outputting red, green, and blue (RGB) light are required in many applications (e.g., light valve projectors). For low brightness light valve projectors RGB light emitting diodes (LEDs) can be used. As the efficiency of green LEDs is currently inferior to red and blue LEDs, the green LED is typically the limiting factor in maximal achievable brightness. RGB Laser light sources can be used to build extremely high brightness light valve projectors. Blue and red laser diode arrays are available as sources for the blue and red light respectively. However, currently green laser diodes have low efficiency and low output power. Frequency doubled green lasers are available at high power, but are much more expensive then red and blue diode lasers.
In order to create cost effective, high brightness, RGB light sources, phosphor conversion technology has been used to convert lower wavelength excitation light like blue or near-UV laser light into higher wavelength light like green, yellow or red light. An RGB light source may be created, e.g., utilizing a red laser diode array, a blue laser diode array, and a phosphor that emits green light when illuminated with blue light (i.e., from the blue laser diode array). The use of green phosphors in RGB light sources may, however, limit the utility of the light source.
When illuminated with an excitation light source, phosphors emit fluorescent light in nearly every direction, significantly growing the étendue of a light source and making it difficult to collect a majority of the emitted light from the phosphor. Using a lens systems to collect a majority of the emitted light will further grow the étendue of the light source. In order to function with the highest possible effectiveness as a light source, the étendue of the light source needs to be smaller than the étendue of the projector. In order to meet this requirement and reduce the étendue, the excitation light source may be limited to illuminating a smaller area (spot size) of the phosphor. However, problematically, the smaller the illuminated spot size, the higher the energy density and the more difficult it becomes to cool the phosphor and maintain the highest conversion efficiency on the phosphor.
In addition to étendue concerns, the use of phosphors in RGB light sources introduces other limitations. For example, light from an excitation light needs to be evenly distributed across a phosphor to avoid the creation of hotspots in the phosphor where locally, due to thermal quenching, the efficiency is significantly reduced. Additionally, light emitted from a phosphor is not polarized and, thus, phosphors cannot be used in applications requiring polarized light.